Several processes for extraction of metals from their oxides have used electrolysis on an industrial scale since the invention of the Hall-Héroult cell for aluminum production in 1886 (U.S. Pat. No. 400,664; herein incorporated by reference in its entirety). When the raw material is water-soluble and the product metal is not very reactive, then this can be done at room temperature in an aqueous electrolyte, e.g. electrolysis of copper chloride to make copper metal and chlorine gas. For others like aluminum oxide electrolysis in the Hall-Héroult cell, it is necessary to dissolve the raw material in a molten salt electrolyte such as cryolite, which in turn requires high temperature cell operation.
It is difficult to find an anode material that exhibits good electrical conductivity, whose reaction with oxygen does not cause a problem for cell operation, and which is not expensive. The Hall-Héroult cell uses graphitic carbon, which adds a consumable material as an operating expense, and whose reaction with oxygen and molten cryolite produces carbon dioxide, perfluorocarbons, and other harmful reaction products.
Other materials include, for example, aluminum bronzes, such as aluminum-copper intermetallic compounds and alloys (U.S. Pat. No. 5,254,232; herein incorporated by reference in its entirety); cermets or ceramic-metal composites (U.S. Pat. Nos. 4,397,729; 5,006,209; each herein incorporated by reference in its entirety); electronic oxides, which are oxide materials with good electronic conductivity, such as nickel ferrite and tin oxide (U.S. Pat. No. 4,173,518; herein incorporated by reference in its entirety); and porous graphite with natural gas reductant (Namboothiri et al., Asia-Pacific J. Chem. Eng. 2007, 2(5), 442-7; herein incorporated by reference in its entirety). The Namboothiri process uses graphite and gas in direct contact with the molten salt, and does not use a liquid metal anode.
The solid oxide membrane (SOM) electrolysis process has provided an alternative electrochemical method for refinement of metal oxides, and sends a pure oxygen gas stream to the anode (see, for example, U.S. Pat. Nos. 5,976,345 and 6,299,742; each herein incorporated by reference in its entirety). The SOM process comprises a solid oxygen ion-conducting membrane (SOM) typically consisting of zirconia stabilized by yttria (YSZ) or other low valence oxide-stabilized zirconia, for example, magnesia- or calcia-stabilized zirconia (MSZ or CSZ, respectively) in contact with the molten salt electrolyte bath in which the metal oxide is dissolved, an anode in ion-conducting contact with the solid oxygen ion-conducting membrane, and a power supply for establishing a potential between the cathode and anode. The metal cations are reduced to metal at the cathode, and oxygen ions migrate through the membrane to the anode where they are oxidized to produce oxygen gas. The first demonstration of the SOM process produced a few tenths of a gram of iron and silicon in a steelmaking slag, and the process has made progress toward the industrial production of other metals such as magnesium, tantalum and titanium (see, for example, U.S. Pat. No. 6,299,742; Pal and Powell, JOM 2007, 59(5):44-49; Metall. Trans. 31B:733 (2000); Krishnan et al, Metall. Mater. Trans. 36B:463-473 (2005); and Krishnan et al, Scand. J. Metall. 34(5): 293-301 (2005); each hereby incorporated by reference herein in its entirety).
The SOM process runs at high temperature, typically 1000-1300° C., in order to maintain high ionic conductivity of the SOM. However, this presents a problem for the anode, which must have good electronic conductivity at this high temperature while exposed to pure oxygen gas at approximately 1 atm pressure.
The best technical approach to date has been to use either an oxygen-stable liquid metal, such as silver or its alloys with dilute copper, tin, etc., or “oxygen stable electronic oxides, oxygen stable cermets, and stabilized zirconia composites with oxygen stable electronic oxides,” as the anode (PCT/US06/027255; herein incorporated by reference in its entirety). If the anode is a liquid metal, then the oxygen produced there accumulates dissolved in the metal until its evaporation rate balances the rate of its production. Liquid metal anodes have the advantage of excellent electronic conductivity (around 10,000 S/cm for liquid silver), simplicity and robustness, and gas permeability is relatively good as long as oxygen bubbles can form easily. However, oxygen-stable liquid metal candidates are typically limited to very expensive silver and gold, and their alloys with very small amounts of other metals.
Thus, there remains a need for more efficient and scalable apparatuses and processes to process metal oxides into pure metals. There also remains a need for stable and inexpensive anode systems to process metal oxides into pure metals. In particular, there remains a need for apparatuses and methods that stabilize anodes in an oxygen generating environment. This invention addresses these needs.